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Unavoidable refractive-index changes induced by the pump beams in DFWM reduce the gain and shift its spectral pass band when the forward and backward pump intensities are unequal. The full linearized DFWM equations also imply large non-phase-conjugate signals in a collinear geometry. These effects influence the onset and spectrum of ’’spontaneous oscillation.’’
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Marburger et al. (1979) studied this question.
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